These days, following the development of the so-called IT industry, remarkable technological innovation has been required in the information recording technology, particularly in the magnetic recording technology. In magnetic disks to be mounted in hard disk drives (HDDs) being magnetic disk devices for use as computer storages or the like, differing from other magnetic recording media such as magnetic tapes or flexible disks, information recording density has been increased rapidly. Supported by this increase in information recording density of the magnetic disks, the information recording capacity of hard disk drives that can be accommodated in personal computer apparatuses has increased remarkably.
Such a magnetic disk is constituted by forming a magnetic layer and so forth on a substrate such as an aluminum-based alloy substrate or a glass substrate. In the hard disk drive, using a magnetic head flying over the magnetic disk rotating at a high speed, information signals are recorded on the magnetic layer as magnetization patterns or reproduced.
In recent years, in such magnetic disks, the information recording density has exceeded 40 gigabits per inch2 and, further, an ultra-high recording density exceeding even 100 gigabits per inch2 is being realized. The recent magnetic disks capable of realizing such a high information recording density have a feature in that, even with a much smaller disk area as compared with those of conventional magnetic disks such as flexible disks, it can store a practically sufficient information amount.
Further, such magnetic disks also have a feature in that, as compared with other information recording media, it is much faster in information recording and reproducing speeds (response speeds) and thus enables real-time writing and reading of information.
Attention has been paid to various features of such magnetic disks. As a result, in recent years, there have been required miniaturized hard disk drives that can be mounted in portable devices, such as so-called portable telephones, digital cameras, portable information devices (e.g. PDA (personal digital assistant)), or car navigation systems, having much smaller housings than those of personal computer apparatuses and required to have high response speeds.
Following the increase in requirement for mounting the hard disk drives in the portable devices (so-called “mobile use”), glass substrates each made of glass being a hard material have been used as magnetic-disk substrates. This is because the glass substrate has high strength and high rigidity as compared with a substrate made of metal being a soft material.
Further, in the case of the glass substrate, the smooth surface can be obtained. Therefore, it is possible to achieve a reduction in flying amount (a reduction in flying height) of a magnetic head that performs recording/reproduction while flying over a magnetic disk, while preventing failures such as head crash and thermal asperity. As consequence, it is possible to obtain a magnetic disk with high information recording density.
However, the glass substrate also has a side of being a brittle material. In view of this, various glass substrate strengthening methods have been conventionally proposed. For example, Patent Document 1 describes a chemical strengthening process in which, as shown in FIG. 3, a glass substrate 101 is immersed for a predetermined time in a solution of a nitrate such as sodium nitrate (NaNO3) or potassium nitrate (KNO3) heated to about 300° C. in a chemical strengthening vessel 102 to replace lithium ions (Li+) at surface layer portions of the glass substrate by sodium ions (Na+) or potassium ions (K+), or replace sodium ions (Na+) at surface layer portions of the glass substrate by potassium ions (K+), thereby forming compressive stress layers at the surface layer portions on both sides so that a tensile stress layer is formed between the compressive stress layers.
On the other hand, Patent Document 2 describes a method of preventing so-called “weathering” that occurs after a chemical strengthening process.
This “weathering” is generally a phenomenon in which when water adheres to the surface of a chemically strengthened glass substrate and carbon dioxide in the air is mixed into this water to form carbonic acid (H2CO3), sodium ions (Na+) in the glass substrate and the carbonic acid react to form sodium carbonate, sodium bicarbonate, potassium carbonate, or the like, so that convex portions are formed on the surface.
Patent Document 2 describes that a glass substrate is immersed in a solution of a molten salt consisting of only potassium nitrate (KNO3) and then immersed in a solution of a mixed molten salt of sodium nitrate (NaNO3) and potassium nitrate (KNO3), thereby suppressing alkaline elution to the glass surface to obtain the glass substrate having high chemical durability.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-121051    Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. Hei 7-223844